1. Field of the Invention
The present invention generally relates to a semiconductor device, more particularly to a complementary metal oxide (CMOS) silicon field effect transistor (FET) having a polysilicon gate electrode with a sub-0.1 xcexcm gate length scale.
2. Background Description
The escalating requirements for high density and performance associated with ultra large scale integration semiconductor devices involves submicron device features, high reliability and increased manufacturing throughput.
Conventional practices are primarily based upon a polysilicon gate CMOS process, in which source and drain regions are formed by implanting impurity atoms in the substrate by using a polysilicon gate electrode as a mask. This has an advantage of doping the polysilicon gate simultaneously when source and drain regions are formed by ion implantation at the same implantation dose. Subsequently, annealing is performed to activate the implanted impurity atoms and also to diffuse the impurity atoms implanted into the polysilicon gate throughout the gate to reach the interface with the gate oxide.
The level of dopant activation in a polysilicon gate is determined by solid solubility of the dopant and the chemical dopant concentration in the polysilicon gate. To maximize dopant activation in a polysilicon gate, the chemical dopant concentration is ideally required to be as high as the dopant solid solubility at the annealing temperature. Here, since the polysilicon gate is doped at the same implantation dose energy with the source and drain regions, the implantation dose and energy for the source and drain regions strictly limit the dopant concentration and the maximum activation level of the polysilicon gate.
In general, diffusion proceeds proportionally to the impurity concentration gradient. Therefore, for devices having a gate length smaller than 0.1 xcexcm, excessively high concentration doping for the source and drain regions causes excessive lateral diffusion of the dopants (e.g., boron, phosphorous) into the channel region. Thus, the dopant concentration of the source and drain regions must be strictly limited below a certain level, and, thus, can not be compromised by increasing the chemical dopant concentration of the polysilicon gate to ideal levels.
This causes insufficient impurity concentration for the polysilicon gate, insufficient activation of a polysilicon gate, and eventually a gate depletion problem when a bias current is applied to the gate to turn on the MOSFET device. This in turn results degradation of the device performance, such as reduction of the output current due to the increased effective gate dielectric thickness.
Additionally, in order to effectively activate source/drain extensions and source/drain regions formed by deep source/drain doping, the thermal cycle for annealing is required to be performed at a high temperature to increase the activation, but must be performed as quickly as possible to avoid excessive dopant redistribution. However, such a short thermal cycle during the annealing step is not sufficient to fully activate the implanted dopants in the polysilicon gate.
Furthermore, the grain size in the polysilicon varies depending on the dopant types and thermal processing sequences. For a P-type MOSFET, boron implanted in the polysilicon gate should not penetrate into the gate oxide while boron diffusion needs to be sufficient to achieve a fully activated P-type gate electrode. Therefore, to fully activate a gate electrode, a precisely calibrated annealing step with a specific temperature and thermal cycle is required depending on the type of the dopant in the polysilicon gate. Also, the optimal thermal cycles for activating a P-type MOSFET is very often significantly different from that of an N-type MOSFET. Thus, if an annealing step is performed to activate source/drain regions only, the gate electrode may not be fully activated.
As a possible alternative, a metal gate has been introduced, but it has been found that it is difficult and costly to achieve a proper work function difference between an N-type MOS transistor and a P-type MOS transistor. Therefore, a metal gate is not considered to be a suitable or successful solution to obtain optimal transistor performance consistent with manufacturing economy.
An advantage of the present invention is to achieve maximized polysilicon gate activation while preventing source/drain regions from being doped at an excessively high impurity concentration, and thereby avoiding lateral encroachment of the dopant into the channel. Another advantage of the present invention is to provide a production-worthy manufacturing method for maximization of gate electrode activation without significant modification of the processing steps and equipments. A further advantage of the present invention is to provide a precisely calibrated annealing step with an optimal temperature and thermal cycle depending on the dopant type implanted in the gate electrode. Furthermore, the present invention provides various manufacturing sequences to achieve the maximum gate electrode activation.
Additional advantages and other features of the present invention will be set forth in part in the description which follows and will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The objects and advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a method for manufacturing a semiconductor device, comprising the step of forming source/drain regions of a first conductivity type in a main surface of a semiconductor substrate, and selectively doping a gate electrode formed on said main surface, so that said gate electrode has an impurity concentration of said first conductivity type higher than that of said source/drain regions.
According to certain embodiments of the present invention, the gate electrode is fully doped by two steps. First, the gate electrode is partially doped by a separate ion implantation step while the source/drain regions are masked. Then, the gate electrode is fully doped when the source/drain regions are formed by ion implantation.
Thus, according to the present invention, the gate electrode is doped at an optimized impurity implantation concentration, independently from the impurity implantation for the source/drain regions. Accordingly, the present invention enables maximization of the gate electrode activation while preventing the source/drain regions from being doped at an excessively high impurity concentration.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.